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.Dd $Mdocdate: August 31 2014 $
.Dt POPEN 3
.Os
.Sh NAME
.Nm popen ,
.Nm pclose
.Nd process I/O
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn popen "const char *command" "const char *type"
.Ft int
.Fn pclose "FILE *stream"
.Sh DESCRIPTION
The
.Fn popen
function
.Dq opens
a process by creating a pipe, forking, and invoking the shell.
Since a pipe is by definition unidirectional, the
.Fa type
argument may specify only reading or writing, not both;
the resulting stream is correspondingly read-only or write-only.
.Pp
The
.Fa command
argument is a pointer to a NUL-terminated
string containing a shell command line.
This command is passed to
.Pa /bin/sh
using the
.Fl c
flag; interpretation, if any, is performed by the shell.
The
.Fa type
argument is a pointer to a NUL-terminated
string which must be either
.Qq r
or
.Qq re
for reading
or
.Qq w
or
.Qq we
for writing.
If the letter
.Qq e
is present in the string then the close-on-exec flag shall be set on the
file descriptor underlying the
.Vt FILE
that is returned.
.Pp
The return value from
.Fn popen
is a normal standard I/O stream in all respects
except that it must be closed with
.Fn pclose
rather than
.Xr fclose 3 .
Writing to such a stream
writes to the standard input of the command;
the command's standard output is the same as that of the process that called
.Fn popen ,
unless this is altered by the command itself.
Conversely, reading from a
.Dq popened
stream reads the command's standard output, and
the command's standard input is the same as that of the process that called
.Fn popen .
.Pp
Note that
.Fn popen
output streams are fully buffered by default.
In addition, fork handlers established using
.Xr pthread_atfork 3
are not called when a multithreaded program calls
.Fn popen .
.Pp
The
.Fn pclose
function waits for the associated process to terminate and returns the
exit status of the command as returned by
.Xr wait4 2 .
.Sh RETURN VALUES
The
.Fn popen
function returns
.Dv NULL
if the
.Xr fork 2
or
.Xr pipe 2
calls fail,
or if it cannot allocate memory.
.Pp
The
.Fn pclose
function returns \-1 if
.Fa stream
is not associated with a
.Dq popened
command, if
.Fa stream
already
.Dq pclosed ,
or if
.Xr wait4 2
returns an error.
.Sh ERRORS
The
.Fn popen
function does not reliably set
.Va errno .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr pipe 2 ,
.Xr wait4 2 ,
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr stdio 3 ,
.Xr system 3
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .
.Sh BUGS
Since the standard input of a command opened for reading
shares its seek offset with the process that called
.Fn popen ,
if the original process has done a buffered read,
the command's input position may not be as expected.
Similarly, the output from a command opened for writing
may become intermingled with that of the original process.
The latter can be avoided by calling
.Xr fflush 3
before
.Fn popen .
.Pp
Failure to execute the shell is indistinguishable from the shell's
failure to execute
.Fa command ,
or an immediate exit of the command.
The only hint is an exit status of 127.
.Pp
The
.Fn popen
argument always calls
.Xr sh 1 .
